As an ink composition filled in a ballpoint pen, water-based ink compositions using water as a main solvent have ordinarily been used in recent years in consideration of an environmental problem and safety to the human body. Of such compositions, those having a shear-thinning viscosity cannot cause bleed of handwriting which will otherwise occur when a low-viscosity water-based ballpoint pen is used.
As a substance (shear-thinning tackifier) giving to the ink composition shear-thinning viscosity, polysaccharides such as xanthan gum can be mentioned as an example (refer to, for example, Patent Reference 1).
The shear-thinning tackifier however sometimes hinders dry-up resistance. Even if an additive, for example, a humectant such as Water soluble organic solvent or solid humectant such as urea is used in combination to improve dry-up resistance, dry-up cannot be prevented fully and in addition, failure in writing such as blur tends to occur owing to an increase in ink viscosity.
Described specifically, excessive addition of urea causes-evaporation of water from the writing tip end to raise the concentration of a water soluble organic solvent. This causes so-called flowering phenomenon, that is, precipitation of the solid content at the writing tip end, which is not favorable visually. At the same time, it causes blur. Moreover, addition of a large amount of a water soluble organic solvent or urea tends to have harmful effects such as occurrence of sag when the ballpoint pen is left with the writing tip end down under high humidity environments.
As described above, the dry-up resistance of an ink composition can be improved by various methods, but none of them can fully solve the problems such as blur of handwriting and sag. Dry-up resistance is an important factor. In particular, when an ink composition is used for a ballpoint pen (capless ballpoint pen) which does not need a cap and has a writing tip end always opened in the air during non-writing time, dry-up resistance is an important factor.
[Patent Reference 1] JP-A-sho 59-74175